1. Field of the Invention
The invention pertains to apparatus and methods for chemically depositing a solid state alkali, preferably lithium, ion conducting electrolyte on a substrate, and methods for incorporating the electrolyte into a battery.
2. Description of Related Art
Lithium ion battery provides the highest energy density and specific energy of any battery chemistry. Hence it is considered as a promising candidate for transportation and stationary energy storage applications. However, dramatic improvements are required in safety, energy density, cycle life and cost before these batteries are adopted for widespread use in transportation. Safety problems arise mainly from the presence of volatile organic solvents and cathode materials, which undergo exothermic reactions under certain operational and abuse conditions, potentially leading to catastrophic thermal runaway. The presence of liquids also causes lithium dendrite growth under conditions of uneven current distributions, especially at high rates of charge/discharge. Finally, traditional Li-ion cell manufacturing is extremely capital-intensive creating substantial financial barriers to scaling manufacturing. The best solution is to use inorganic, solid-state components, which eliminate the problems caused by liquid electrolyte systems. In addition to improved safety advantages, they also provide the flexibility to use higher energy cathode materials, substantially increase energy density, and greatly extend cycle life.
Though thio-LISICON solid state electrolytes of the form LiSP, LiSiPS, LiGePS, or in general LixM1-yM′yS4 (M=Si, Ge, and M′=P, Al, Zn, Ga, Sb) have been found with ionic conductivity comparable to that of liquid electrolyte [see Masahiro et al., Solid State Ionics 170:173-180 (2004)], the method of growth is often expensive and cumbersome, and the resulting electrolyte materials are in pellet, ceramic/glass plate, or powder forms, making their integration in a large format solid state lithium ion battery difficult to implement.
Seino et al., in U.S. Pat. Appl. Pub. 200910011339A1 disclose a lithium ion-conducting solid electrolyte comprising high purity lithium sulfide (Li2S), diboron trisulfide (B2S3), and compound represented by LiaMOb; where LiaMOb is either lithium silicate (Li4SiO4), lithium borate (Li3BO3), or lithium phosphate (Li3PO4). The powder of these compounds were mixed together in the right proportion and pelletized. The pellets were subjected to 800° C. for 4 hours for melt reaction. After cooling the pellet was further subjected to heat treatment at 300° C. to form high lithium ion conducting solid electrolyte.
Kugai et al., in U.S. Pat. No. 6,641,863 used vacuum evaporation, vacuum laser ablation, or vacuum ion plating to deposit a thin film of solid electrolyte with preferred thickness of 0.1 to 2 μm on the anode. The film electrolyte is obtained by evaporating a mixture of Li2S, A, and B compounds; where A is GeS2, Ga2S3, or SiS2, and B is Li3PO4-xN2x/3, Li4SiO4-xN2x/3, Li4GeO4-xN2x/3 (with 0<x<4), or Li3BO3-xN2x/3 (with 0<x<3). The electrolyte film is deposited on the anode to block the Li dendrite growth in liquid electrolyte based lithium ion secondary batteries. In-situ or post deposition heat treatment at temperatures ranging between 40 to 200° C. is done to increase the lithium ion conductivity of the solid state electrolyte film to a value that is comparable to that of liquid electrolyte.
Minami et al., [see Solid State Ionics 178:837-41 (2007)], used mechanical ball milling to mix selected proportions of Li2S and P2S5 crystalline powders at 370 rpm for 20 hours. The finely milled powder mixture is then heated in a sealed quartz tube at temperature of 750° C. for 20 hours to form a molten sample. This was quenched with ice to form 70Li2S.30P2S5 glass. The glass was then annealed at 280° C. to form 70Li2S.30P2S5 ceramic glass (Li7P3S11) with an ionic conductivity of about 2.2×10−3 S cm−1.
Trevey et al. [see Electrochemistry Communications, 11(9):1830-33, (2009)] used heated mechanical ball milling at about 55° C. to grind and mix the appropriate proportion of Li2S and P2S5 crystalline powders for 20 hours to form a glass ceramic powder of 77.5Li2S.22.5P2S5 having 1.27×10−3 S·cm−1 ionic conductivity. The powder is then pelletized for use in a battery.
The starting raw materials in all these cases are powders of various compounds of elements constituting the electrolyte. In one case, these are used in expensive vacuum systems to deposit thin films of the electrolyte. The use of this process to deposit 0.1 to 2 μm film to block lithium dendrite formation on anode in a liquid electrolyte based lithium-ion battery will incur some price penalty; however, its use in depositing a thicker film suitable for a large format all-solid-state lithium ion battery will be uneconomical. In the other case, the use of ball milling to obtain finer powder appears cumbersome. The integration of glass ceramic electrolyte, obtained from powder melting at high temperature and quenching, in the overall battery fabrication steps is not trivial and may be impossible. However, the option where melt quenching is omitted and pelletization of combined anode, electrolyte, and cathode to fabricate the battery is feasible and slightly less expensive. But one can foresee a bulky battery, perhaps in a coin cell format, with lower energy per unit mass.
What is needed, therefore, is a flexible and economical method for growing thin or thick, high lithium ion conducting solid state electrolyte films where the growth starts from atomic level mixing of most or all of the constituent elements. To reduce the overall battery fabrication cost, the method should also lend itself to seamless integration with other process steps in battery fabrication.